Structural transformations in cubic ZrMo2O8 at high pressures and high temperatures

High-pressure high-temperature syntheses using the gamma-ZrMo2O8 (Pa (3) over bar, Z = 4) starting material with the structure of the beta-ZrW2O8 type are carried out in a multi-anvil apparatus. The recovered products are examined with powder x-ray diffraction at ambient conditions. The sample obtained at 1.3 GPa and 298 K is partially amorphous and contains gamma-ZrMo2O8. Up to the onset of amorphization at 1.3 GPa, the recovered products from near room and high temperatures are alpha-ZrMo2O8 (P (3) over bar 1c, Z = 6) and beta-ZrMo2O8 (C2/c, Z = 4), respectively. In the structure of alpha-ZrMo2O8, layers of ZrO6 Octahedra and MoO4 tetrahedra are joined together by van der Waals forces. beta-ZrMo2O8 is built of infinite ribbons of edge-sharing ZrO8 polyhedra joined by MoO5 pyramids. The crystalline compounds synthesized at pressures above 1.3 GPa and at high temperatures. originate from the pressure-induced amorphous phase of gamma-ZrMo2O8. The higher the pressure during the synthesis the higher the temperature necessary to obtain a recoverable crystalline product. Up to about 4 GPa, the synthesized material is beta-ZrMo2O8. The recovered crystalline phases from above 4.0 GPa are the ZrO2-MoO3 decomposition products. One of them is the high-pressure high-temperature form Of MoO3 (P2(1)/m, Z = 2), with all the Mo atoms in the octahedral coordination. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.